ÒLIESEGANG
BANDINGÓ
Irregular, reddish or brownish or orangish-brown iron
banding is commonly seen in many porous rocks, particularly sandstones and
pebbly sandstones. These have been almost universally referred to as
ÒLiesegang BandingÓ, representing precipitation lines of iron-rich minerals
(e.g., hematite, limonite, goethite, etc.) at & along groundwater chemical
interfaces. But, according to Neil Wells of Kent
State University, the original concept of Liesegang banding (Liesegang, 1896)
does not match up with what is seen in the rock record (Wells et al., 2003).
True Liesegang banding refers to parallel bands of
precipitate formed by diffusion along a single chemical gradient during one event.
What's seen in the rock record often consists of sets of irregularly concentric
iron bands, with different sets of bands quite frequently oriented in different
directions, and showing cross-cutting and dissolution of older sets. Iron
banding in the rock record is clearly the result of numerous precipitation
events over long periods of time by moving groundwater (Wells et al.,
2003). Iron mineralization along these bands appears to be induced by the
presence of either a redox interface (change from reducing to oxidizing
conditions in the groundwater) or a pH interface (change in acidity).
Since Neil Wells is the first (apparently) to point
out that what geologists have been calling Liesegang banding really isn't, a
renaming seems to be in order. It was jokingly suggested in 2003 that the
iron banding discussed above be called ÒWells BandingÓ. I'm all for that.
ÒLiesegang bandingÓ developed in a volcanic
tuff/breccia (8.6 cm across).
ÒLiesegang bandingÓ developed in a quartzose
sandstone (= ÒScenic SandstoneÓ) (~20.5 cm across).
References:
Neil Wells (pers. comm., 2003)
Liesegang, R.E. 1896. Ueber einige
Eigenschaften von Gal-lerten [On some properties of gelatin]. Naturwissenschaftliche
Wochenschrift 11: 353-362. (see also: Liesegang, R.E. 1945.
Geologische BŠnderungen durch Diffusion und KapillaritŠt [Geologic banding by
diffusion and capillarity]. Chemie der Erde, Zeitschrift der
Chemischen, Mineralogie, Petrographie, Geologie und Bodenkunde 15:
420-423.)
Wells, N.A., D.A. Waugh & A.M. Foos.
2003. Some notes and hypotheses concerning iron and iron remobilization
features in the Sharon Formation (Summit County, Ohio). in
Pennsylvanian Sharon Formation, past and present: sedimentology, hydrogeology,
and historical and environmental significance, a field guide to Gorge Metro
Park, Virginia Kendall Ledges in the Cuyahoga Valley National Park, and other
sites in northeast Ohio. Ohio Division of Geological Survey Guidebook
18: 33-37.